This invention relates to an apparatus and method employing Spread Spectrum (SS) signal structures for the operation of one or more touch-input devices on a touch-sensing system.
A touch system consists of two parts, namely one or more touch-input devices and a touch-sensing architecture. These two parts themselves can consist of hardware and/or software structures to realize their functionality.
In this document, a touch-sensing tablet (termed touch screen hereafter) can be regarded as a touch screen, a digitizer, a writing panel, a modified mouse pad, or the like. A touch-input device can be regarded as a stylus, a pen, a rotary knob, a mouse, a slider (fader), and the like. The system operation is defined as, but not limited to, one or any possible combination of the following functionalities, namely a touch screen (or its equivalent) that identifies, tracks, or communicates with one or more touch-input devices.
Touch screen technologies known in the prior art are most easily differentiated according to their system infrastructures. They are traditionally classified into resistive, or pressure sensing; capacitive; surface acoustic wave; ultrasound; and electromagnetic (EM) wave systems. The touch screen technologies of concern here are capacitive and direct-contact touch screens which only involve a form of electrical contact with the touch surface.
In capacitive systems, the screen assembly includes a sensing layer that is capable of storing electrical charges. Electrical sensors located at the boundaries of the touch screen apply an electrical field that is distributed across the touch screen surface, forming, in effect, a distributed capacitor. In a passive touch, a human finger or a conductive device touches the screen and draws a current from the sensors. The differential in the current flows in the boundary sensors corresponds to the position of the touch on the screen. For this reason, passive capacitive touch screens do not work well, if at all, when used with a non-conductive device, such as a gloved hand or an inert stylus. In an active capacitive system, an active device emits an excitation signal at the touch point, injecting current into the sensors, and the current is measured to determine the touch position. Active capacitive systems usually have an improved touch resolution over passive systems, due to the fact that an active device provides an improved Signal-to-Noise Ratio (SNR) compared to passive systems. Capacitive systems are very durable, with high screen clarity.
In direct-contact touch systems, the screen assembly includes a sensing layer that is an open conductive contact surface. Examples of this surface are a resistive Indium Tin Oxide (ITO), Tin Oxide (TO), or any other resistive non-transparent surface. Electrical sensors located at the boundaries of the touch screen are sensitive to electrical energy coming in contact with the surface thereby applying a signal received at each of the sensors. The surface is initially at ground potential and an electrically charged stylus supplies current is drawn from the stylus to the contact surface. The differential in the voltage levels in the boundary sensors corresponds to the position of the touch on the screen. Similar to capacitive, direct-contact touch screens do not work well, if at all, when used with a non-conductive device, such as an inert stylus. In an active direct-contact system, an active device emits an excitation signal at the touch point, injecting a signal into the sensors, and the signal amplitude is measured to determine the touch position. Active direct-contact systems also have a high touch resolution, and are very durable, with high screen clarity. A direct-contact system can operate with low active voltages allowing methods for self-powering with EM fields (i.e. battery-free), and tether-free stylus operation (i.e. no ground cable).
In the prior art, the number of touch-input devices allowed in a touch system is generally limited to one. However, in U.S. Pat. Nos. 6,005,555, 6,020,849, and other similar patents, methods of operating multiple touch-input devices are addressed, with each device designed to work on single or multiple narrowband channels.
Concerning information encoding, U.S. Pat. No. 5,247,138 describes a cordless digitizer stylus that transmits encoded signal to a touch-sensing tablet. This signal contains information bits relating to the touch-input device such as on-off status of the switches, position of the device, etc. These information bits are coded by a binary code at a particular frequency, and the information carrying signal disclosed is a narrowband signal.
In U.S. Pat. No. 6,005,555, a touch system with two carrier frequencies f0 and f1 is disclosed. Information bits of the system are commands from the touch-sensing tablet to the touch-input devices and data bits from the devices to the tablet. The system signal spectrum consists of two discrete information spectra, centered at two carrier frequencies f0 and f1. No signal with bandwidth wider than the information bandwidth is used. Similar disclosures can be found in other patents regarding touch screens.
In U.S. patent application Ser. No. 09/877,611 the concept of CDMA signaling is used to describe how a plurality of devices can be simultaneously used on a touch screen surface, allowing all devices to be separately locatable. This disclosure discusses the use of CDMA applied to capacitive and direct-contact touch systems (and other touch systems such as acoustic, ultrasonic surface wave, EM, etc.) where a plurality of electrical contact devices are locatable on a touch surface.
It should be emphasized that touch systems of the prior art, including the above mentioned patents, are regarded as narrowband systems. That is, these systems have their signal bandwidth at no wider than the information bandwidth, as shown in FIG. 1A. There is no wideband encoding for the system information bits or carriers in these patent disclosures.
In summary, the signal spectra of the above mentioned narrowband systems are the combination of the discrete information spectra at individual carrier frequencies. Their signal energy is confined within these discrete information spectra. No extra bandwidth other than the information spectra is occupied. These narrowband systems are significantly different from wideband systems, namely spread spectrum (SS) systems, of this invention.